While the currently accepted answer talks about inertia briefly, I thought I'd expand on it. A couple years ago I made this axe:
Due to it's small size, it weighs only a gram or two. No matter how hard you swing it, it would not penetrate human skin (though that may also say something about my poor sharpening skills). I also made a bunch of tiny swords, and they were more effective because you could actually push them through things using physical strength.
But this inertia limitation is true of nearly all scaled down weapons: How much would an arrow weigh? You can try this yourself. Grab a sewing needle, hold it 30cm above the palm of your hand, and drop it point first. It will bounce right off your skin. [disclaimer: tested with a normal size sewing needle of ~5cm length].
As pointed out @nagamani, a bullet would weigh very little, but here's a video of a 2" long gun being fired and I wouldn't want to be in the way of that! If travelling fast enough, a small weapon can still be painful or deadly. In the case of an axe, you're limited by the radius from the pixie and how fast they can swing it, but for other weapons, you are limited by other factors.
While your pixies don't have firearms, a medieval technology level does give them access to bows. A couple months back I carved half a dozen bows from bamboo barbecue skewers:
These can shoot another skewer well into corrugated cardboard, and years ago I made a 12" bow from a hardwood that could lob sharp pointy things about a dozen meters. This is possible because the velocity of the arrow is constrained by the elasticity of the bow, and the critical velocity of wood is relatively constant. If you need a testament to the hazards of these things: a few years ago, I managed to sink about 5mm of an unsharpened wooden kebab skewer into one of my fingers from a rubber-band powered crosbow-gun-thing (it was the pointy end, but from the packet, a skewer isn't particularly sharp).
What this means is that a small bow and arrow may be usable by a pixie. @ChrisW calculated that a 6" pixie capable of lifting it's own weight can exert about 100 grams of force. I don't have any little bows on me currently (perhaps I'll make one tomorrow), but I think this is enough for a gang of pixie-longbow-archers.
Considering that your pixies have to hunt to survive, I suspect they would know a fair bit about poisons because just about everything they encounter will be larger and tougher than them. If this is the case, a single bow-wielding pixie is probably capable of taking down a human from several feet range if they have a sufficiently powerful toxin.
If your pixies are able to scavenge things off the human world, I could suggest raiding a craft/diy store and taking a stack of #11 exacto blades:
These things are super-sharp, and would be well wielded by fairies either as daggers (see this book cover), or as tips for spears, arrows etc. They come in handy packs of ten, enough for a whole gang of pixies....
One final thing. A couple years back, I encountered an oystercatcher (type of bird) that was protecting it's (ground based) nest. Somehow or other, we angered it, and it stalked towards us. Yes, it's only a couple inches high, and none of the photos make it's beak look particularly sharp, but when it's looking at you aggressively it's quite a scary thing. In that case, it's helped by the fact that a bird can fly up at you so it's not quite equivalent to a pixie, but if I were approached by a knife, spear or bow wielding pixie, I would suggest to be very polite to it. While half a dozen tiny cuts won't kill you (probably) they sure would hurt.